Since LEDs (light emitting diodes) have reached high luminous efficacy and the ability to generate high luminous flux, the interest for their use in indoor and outdoor lighting, such as street lighting, has grown very fast. One of the major challenges in using LEDs in applications such as street lighting, is the design of an optical system necessary to achieve an efficient illumination and comply with all requirements and regulations. Lighting fixtures based on LEDs can be very efficient if special attention is paid to the optical system. In order to achieve an efficient illumination over the illuminated surface or area, a uniform light pattern must be generated by the lighting fixture preventing excessive light in areas near the lighting fixture commonly known as hot spots.
Most of the lighting fixtures based on LEDs for street lighting for example, use complex refractive (lenses) or reflective (reflectors) secondary optics that are made for a specific type of LED to achieve a uniform illumination pattern. These secondary optics are specially designed in order to be able to direct light emitted by the LED so as to generate a complex illumination pattern. Commonly, complex secondary optics are used in arrays where each LED has its own individual lens or reflector. One of the major drawbacks of this type of optics is that they usually are limited to only one type of LED from a specific manufacturer. This is because LEDs from each manufacturer are physically different, and many factors such as light emitting area, size and mechanical properties of the LED have to be taken into account when designing this type of optics. This means that once the optical system is developed, a lighting fixture designer will be unable to change from one type of LED to another without redesigning the whole optical system. Moreover, it is impossible to design this type of secondary optics for some LEDs in the market due to their large emitting area which would require a very big lens or reflector making the solution impractical.
On the other hand, simple secondary optics for collimating light in only one direction are easier to develop and are widely available in the market for many types of LEDs. Secondary optics of this type are gaining popularity in applications such as spotlights, downlights, flashlights and many other, where it is not required to generate a complex illumination pattern. In street lighting applications, simple secondary optics can be used in special configurations with different aiming angles for each lens or reflector or along with other type of light shaping method in order to provide an illumination pattern adequate for the application. This gives freedom to lighting fixture designers to use different types of LED by just changing the simple secondary optics without redesigning the main optical system.